The assay of soluble endothelial protein C receptor (sEPCR) is useful to monitor effective thrombin levels and a hypercoagulable state. An assay for sEPCR is therefore useful to monitor ongoing effectiveness of anticoagulant therapy. A sEPCR ELISA assay is particularly useful for this purpose. A state of hypercoagulability in patients or normal individuals can also be identified by such an assay.